In recent years, an exposure apparatus is required to have a higher resolution with further miniaturization of a semiconductor element. To improve the resolution, a decrease in wavelength of an exposure light source or an increase in the projection optical system numeral aperture (NA) is demanded. Since the increase in NA increasingly reduces the depth of focus, still higher-accuracy exposure techniques are desired.
Conventionally, a step-and-repeat exposure apparatus (to be referred to as a “stepper” hereinafter) is employed as an exposure apparatus for the manufacturing process of a semiconductor device or the like. This exposure apparatus sequentially exposes a plurality of exposure regions on a wafer with the pattern of a master (reticle) via a projection optical system while moving the wafer stepwise. However, as the size of a wafer becomes large, and the integration degree of a semiconductor element increases, conventional steppers cannot cope with an increase in exposure area and pattern formation accuracy.
For this reason, the mainstream of exposure apparatuses has shifted from a conventional stepper to a step-and-scan exposure apparatus (to be referred to as a “scan exposure apparatus” hereinafter) which repeats exposure transfer operations in a plurality of regions on a wafer by repeatedly performing step movement and scanning exposure. In a scan exposure apparatus, a wafer and a reticle are respectively chucked by a wafer stage and a reticle stage, and exposure is performed while repeating scanning of both stages relative to each other.
There are two reasons why scan exposure apparatuses have come into wide use. The first reason is that each of them can perform exposure with a micropattern at high accuracy and at a wide field angle because its slit limits exposure light to the part relatively close to the optical axis of the projection optical system. The second reason is that it can perform fine focus driving in a shot because the wafer stage is driven using a plurality of focus measurement values at a plurality of points in a shot.
In the above scan exposure apparatus, focus measurement must inevitably be performed at a plurality of points during stage scanning. For this reason, it is particularly important to determine timings at which focus measurement operations are performed during stage scanning in different shots on a single wafer.
For example, a semiconductor device is formed not by a single exposure operation but by exposing the same position a plurality of number of times using different reticles. Accordingly, to form a micropattern at high accuracy, focus measurement must be performed at the same position each time in a plurality of exposure processes. Assume that a wafer is processed to have a stepped shape as shown in FIG. 2 in the 1st to Nth exposure processes. In FIG. 2, the horizontal axis represents a position in a shot, and the vertical axis schematically represents the stepped shape of the wafer in the shot. At this time, assume that the timing of focus measurement during scanning at each shot in the (N+1)th exposure process shifts from those in the 1st to Nth focus exposure processes. In this case, focus driving is performed by, e.g., focus measurement of a B point instead of an A point, thereby exposing each shot at a position with a defferent offset relative to the exposure image plane. As a result, in a conventional exposure apparatus, such variations in exposure accuracy lead to a decrease in focusing accuracy, thereby reducing the yield of chips.